1. Field of the Invention
The present invention relates to an optical time compression multiplexing transmission system.
2. Background Art
Studies of various optical subscriber systems to realize FTTH (Fiber To the Home) are presently being conducted in every country.
As one of the systems to realize this FTTH, studies and developments of a passive double star (PDS) optical subscriber system are being conducted. In the PDS optical subscriber system, an optical star coupler, which is a passive optical device, is arranged at branch points on optical transmission lines. Then, time division bidirectional communication is carried out between equipment at a central office and more than one piece of equipment at subscriber premises.
In this system, time division bidirectional communication is generally realized in a single optical fiber by multiplexing downloaded optical signals from equipment at a central office to more than one piece of equipment at subscriber premises and uploaded optical signals from these pieces of equipment at subscriber premises to the equipment at a central office in time domain with TCM (Time Compression Multiplexing). Moreover, as a method of multiplexing optical signals to more than one piece of equipment at subscriber premises, a TDMA (Time Division Multiple Access) method which multiplexes these signals in the time domain is used.
In an optical transmission line, reflection occurs at parts, such as optical connectors and so on, which comprise a transmission line. Optical reflection occurs by returning a reflection to a transmission terminal at a time which is equivalent to a round trip delay time from the transmission terminal to where the optical signal was transmitted, to the reflection point, and back to the transmission terminal.
In bidirectional transmissions in a single optical fiber by the TCM-TDMA method, the following problems occur due to the optical reflection which degrade the receiving characteristics.
(Problem 1) Convergence on the optical reflection of an ACG (Automatic Gain Control) amplifier:
Generally, an optical receiving circuit of an optical transmission system is equipped with an automatic gain control amplifier to keep output amplitude at an appropriate level by controlling a degree of amplification according to the input level of received optical signals. Regardless of whether the received light is an optical reflection, the automatic gain control amplifier controls a degree of amplification according to the input level of the received light/optical signal.
Therefore, if the power of an optical reflection is high, and when using the gain which was controlled according to this optical reflection, the signal which should originally be received and follows the optical reflection cannot be sufficiently amplified. For example, when the electrical amplitude level of an optical reflection after changing the optical reflection into an electrical signal is greater than twice the electrical amplitude level of the receiving optical signal which follows the optical reflection, a problem of mistaking a signal which was originally "1" as the signal "0" occurs.
(Problem 2) The missynchronization of the optical reflection:
When a bit pattern which is identical with a frame pattern of a regular receiving optical signal exists in an optical reflection, frame synchronization will mistakenly be established by the optical reflection and the regular received optical signal which follows the optical reflection will probably not be received.
One of the solutions to the above-mentioned misconvergence of the automatic gain control amplifier is to set a guard time between sending and receiving optical signals to be sufficiently longer than the time constant of gain control of the automatic gain control amplifier. In this way, the gain of the automatic gain control amplifier can be made to converge during the guard time after the optical reflection passes.
However, the time constant of the gain control is set sufficiently long compared with the time width of a bit signal to prevent the gain from being changed by noise influence and/or "0"/"1" pattern effect of receiving signals. Therefore, it becomes necessary to set the length of the guard time to be very long so that the length of the guard time is set to be sufficiently longer than the time constant of the gain control. As a result, the time area for sending and receiving optical signals narrows and transmission efficiency decreases.
In addition, the above-mentioned guard time is a constant time which is provided to secure idle time which is necessary to change the operation condition between transmission and reception. This guard time is set respectively at both the central office and the subscriber premises. During this guard time, no optical signals are transmitted from either piece of equipment. That is, the output level from each piece of equipment during the guard time is continuously "0".
On the other hand, another measure to prevent the missynchronization with the optical reflection is to make the frame pattern length sufficiently long. This can lower the probability that a bit pattern in the optical reflection is identical with a frame pattern of a regular receiving optical signal. However, there is a problem in that the transmission efficiency decreases in this case too.
The following references show examples of the conventional art in automatic gain control amplifiers which are used by the subscriber systems. However, there are some problems in the conventional techniques described in these references. No consideration is given to the optical reflection whose occurrence cannot be controlled, or even if consideration is supposed to be given, the configuration is complicated or transmission efficiency has been decreased.
Conventional Art Reference 1) Fumio Mano, et al., "A study of fast-response AGC circuit for passive double star (PDS) optical subscriber systems", the electronic information communication society, Meeting, 1991, B-602.
Conventional Art Reference 2) Japanese Patent Application, First Publication, Sho 57-136831, "Receiving AGC control method."
Conventional Art Reference 3) Japanese Patent Application, First Publication, Hei 1-137752, "Receiving circuit."